In many situations, radio transmitters are identified by special identification codes. These codes are typically transmitted with the other communications a transmitter is sending. In many other situations, however, there is interest in identifying transmitters that are not deliberately identified. For example, it can be useful to have an apparatus and means for identifying radio transmitters that are operating clandestinely or in such a manner as to interfere with the transmissions of others.
Previous attempts to characterize radio transmitters have been based on using the signal amplitude of the transmitter as a function of time. Amplitude-based techniques, however, are susceptible to multipath propagation and path loss. These transmission phenomena can distort the received signal amplitude independently of any transmitter characteristics. Time-history amplitude characterization is also typically used on a transmitter which is already operating and is, therefore, fully warmed up. A warmed-up transmitter is relatively stable and is, accordingly, more difficult to characterize.
It has been determined that the phase or frequency (i.e., time rate of change of phase) response of a transmitter is essentially insensitive to some forms of multipath and path-loss propagation problems that plague amplitude-based signal characterization techniques.
It is therefore desirable to have a method and apparatus for characterizing a radio transmitter that is based on the transient phase history of the transmitter as it is being turned on.